1. Field of the Invention
The present invention relates to a pneumatic cylinder having a cushioning mechanism and a method of cushioning the pneumatic cylinder, and more particularly to a pneumatic cylinder having a cushioning mechanism, with cushioning chambers defined at opposite ends of the pneumatic cylinder, wherein air supplied under pressure to move a piston in one direction is branched, introduced, and stored under pressure in one of the cushioning chambers at a time, and when the piston is moved in an opposite direction, the stored air under pressure is utilized to dampen the movement of the piston, and also to a method of cushioning the piston in the pneumatic cylinder.
2. Description of the Prior Art
Many pneumatic cylinders have a cushioning mechanism to provide against a load having large inertial energy for dampening an inertial force to reduce any undesired shock when the piston reaches a stroke end.
FIG. 1 of the accompanying drawings illustrates a conventional cushioning mechanism in a pneumatic cylinder. The pneumatic cylinder, designated at 2, has a head cover 4 with an inlet/outlet port 6 defined therein. The head cover 4 also has an inner cavity 8 with a cushioning packing 10 fitted in an end thereof. An adjustable needle point valve 12 is disposed in the head cover 4 in diametrically opposite relation to the inlet/outlet port 6, and has a needle point positioned in a bent passage 14 extending from a bore in the cylinder 2 to the cavity 8.
A piston rod 16 is axially movably disposed in the cylinder 2 and supports on an end portion thereof a piston 18 slidably mounted in the cylinder 2. The piston 18 and the cavity 8 jointly define a cushioning chamber 24 therebetween. A seal ring 20 is fitted in and extends along an annular groove defined in a central peripheral surface of the piston 18. A cushioning ring 22 is mounted on the end of the piston rod 16.
When the piston rod 16 is axially moved in the direction of the arrow A to a broken-line position, air flowing from the cushioning chamber 24 is restricted by the needle point valve 12 to reduce the speed of travel of the piston 18 at a stroke end thereof. More specifically, the distal end of the cushioning ring 22 is brought into contact with the cushioning packing 10, and the fluid under pressure from the cushioning chamber 24 is restricted by the passage 14 and the needle point located therein. As a result, the pressure in the cushioning chamber 24 is increased as the piston 18 is moved, thereby providing a cushioning action. The cushioning force available can be adjusted in magnitude by adjusting the needle point valve 14 to vary the cross-sectional area of the passage 14.
However, the cushioning mechanism of the foregoing construction has the following problems: As illustrated in FIG. 2, the speed of operation of a pneumatic cylinder is generally determined by the difference between a driving force F which displaces the piston itself and a braking force S tending to limit the movement of the piston. In a meter-out system, it is customary to regulate the damping force by keeping the driving force F substantially constant while changing the braking force S. Therefore, where the piston reciprocates at a higher speed, air has to be discharged under a lower pressure. More specifically, since the conventional cushioning mechanism dampens the reciprocating movement of the piston by using the discharged air pressure which produces the braking force S, the pressure of air entrapped in the cushioning chamber 24 becomes progressively lower as the speed of the reciprocating movement of the piston 18 goes higher, with the consequence that the ability to absorb or dampen the kinetic energy produced by the reciprocating movement of the piston 18 is reduced, or a desired damping capability is not available.
FIGS. 3(A) and 3(B) illustrate the correlations between the piston stroke and the air pressure, plotted when the piston reciprocates at lower and higher speeds. As the air pressure supplied to the cylinder 2 is progressively increased, the piston 18 is axially displaced and the pressure of air discharged out of the inlet/outlet port 6 is progressively reduced. When the cushioning ring 22 is moved into contact with the cushioning packing 10, compressed air from the cushioning chamber 24 flows through the passage 14 into the cavity 8 from which air is discharged out of the inlet/outlet port 6. The rate at which the pressure of entrapped air is reduced is not greatly reduced from the time the cushioning ring 22 contacts the cushioning packing 10 on. At a lower speed of reciprocating movement of the piston 18 as shown in FIG. 3(A), as the piston stroke increases further, the cushioining pressure is quickly built up as indicated by the hatched area. Accordingly, the pneumatic cylinder achieves a sufficient dampening capability.
When the speed of reciprocating movement of the piston 18 is increased as shown in FIG. 3(B), the pressure of discharge air is abruptly reduced as compared with the supplied air pressure. The pressure of air entrapped in the cushioning chamber 24 is quite low at the time when the cushioning ring 22 contacts the cushioning packing 10. Therefore, the cushioning pressure is much smaller as compared with the cushioning pressure shown in FIG. 3(A), failing to provide a desired dampening capability. The lack of enough dampening forces would allow the piston to be forced against the head cover under a large inertial force, resulting in damage to the head cover or other unwanted accidents. Desigated in FIGS. 3(A) and 3(B) at .DELTA.P is the difference between the supplied air pressure and the discharged air pressure at the end of the piston stroke.